We will utilize a newly-designed method to make rapid sequential determinations of cell volume in the toad urinary bladder and other high resistance sodium transporting epithelia. This involves mounting the tissue in a flow chamber in which the thickness of the mucosal chamber is 25 microns and the serosal 75 microns, thereby allowing excellent optics for microscopic observations. The tissue will be observed with interference contrast microscopy, and the focusing knob interfaced with a computer which will be programmed to change focus at predetermined intervals. At each interval a video image will be obtained and stored on video disc. Subsequently each image will be retrieved for viewing. The cell borders will be scanned with cursors such that the X and Y coordinates at each point will be stored. From these data the area of each cell at each "optical slice" can be calculated. Subsequently the volume can be estimated from the sequential areas. We will then determine cell volume as a function of sodium transport under a variety of conditions in order to determine the factors which control both.